At present, research and development have been made vigorously regarding various display devices and, among all, organic electroluminescence devices (hereinafter simply referred to as “organic EL device”) have attracted attention as prospective display devices in the next generation since they may obtain light emission of high luminance at low voltage.
Since the organic EL device has higher response speed compared with liquid crystals used so far and emits light by itself, it does not require a back light as in existent liquid crystals and may form a flat panel display of an extremely reduced thickness. The organic EL device is a light-emitting device utilizing the electroluminescence phenomenon and, while it is identical with LED in view of the principle, it has a feature in that an organic compound is used as a light-emitting material.
As an example of organic EL devices using such an organic compound as a light-emitting material, an organic EL device utilizing a multi-layered thin film by a vapor deposition method has been reported. For example, light emission characteristics are improved remarkably compared with existent mono-layered device by using tris(8-hydroxy quinolinate-O,N) aluminum (Alq3) as an electron-transporting material and laminating it with a hole-transporting material (for example, aromatic amine compound).
Then, while it has been vigorously studied for the trend of applying such an organic EL device to multi-color displays in recent years and, for developing a multi-color display of high function, it is necessary to improve the characteristics and the efficiency of the light-emitting devices for respective colors of red, green and blue as three primary colors of light.
As means for improving the characteristics of the light-emitting device, it has also been proposed to utilize a phosphorescent material to a light-emitting layer of an organic EL device. The phosphorescence emission is an emission phenomenon from an excited triplet state caused from the excited singlet state by radiationless transition referred to as an inter-state crossing, and it is known to show higher quantum efficiency compared with the fluorescence emission as an emission phenomenon from the excited singlet state. It is expected that a high luminous efficiency may be attained by using the organic compound showing such a property as the light-emitting material.
For the organic EL devices using such phosphorescent material, those devices using various complexes with iridium as the center metal have been developed up to the present, and the development for complexes using platinum as the center metal has also been progressed. Among them, as an organic EL device using red phosphorescent material, a device of using (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphynate-N,N,N,N) platinum (II) (Pt(OEP)) for the light-emitting layer has been reported (Patent Document 1).
However, while the platinum complex is a red phosphorescent material of high color purity, the external quantum efficiency is about 4% and further improvement for the luminous efficiency is demanded. Further, it has also been reported that an ortho-metalated platinum complex using a compound having an arylpyridine skeleton as a ligand and using platinum as the metal is useful as a phosphorescent material (Patent Document 2) and a platinum complex using a biaryl skeleton compound as a ligand has also be reported (Patent Document 3).
As described above, various studies have been made vigorously for practical use of display devices in the next generation and, among them, organic EL devices using the phosphorescent material is particularly highlighted with a view point of improving the characteristics of the devices. However, the study has been quite primitive and includes various subjects such as optimization of light emission characteristics, luminous efficiency, color purity, and structure of the devices. For solving such subjects, it has been demanded for the development of novel phosphorescent material and, further, for the development of efficient method of supplying the material.
Patent Document 1: Specification of U.S. Pat. No. 6,303,238
Patent Document 2: JP-A No. 2001-181617
Patent Document 3: JP-A No. 2002-175884